Constraints on Geodynamic Setting of East Antarctic Orthopyroxene Granitoids Intrusions

Abstract

Orthopyroxene granitoids (designated as charnockites or granitoids of the charnockite series) are characterized by varying silica and felsic mineral content, with the orthopyroxene (hypersthene) phase as the main characteristic mafic mineral component. These rocks are abyssal and represent parts of intrusions or form their own igneous complexes at the depth of the lower crust under CO₂-undersaturated and dry conditions, which can be achieved within any tectonic setting. Therefore, petrogenetic models for orthopyroxene granitoids as well as for all types of granitoids from different locations are sufficiently applicable for a reconstruction of paleogeodynamic conditions in a certain region. The East Antarctic shield is characterized by Archean blocks embedded within orogens of Mesoproterozoic, Late Mesoproterozoic‒Early Neoproterosoic and Late Neoproterosoic‒Early Paleozoic age, and syn- and post-orogenic intrusions composed of orthopyroxene granitoids and related rocks are widely spread and form a prominent volume of the East Antarctic orogens. According to paleogeodynamic reconstructions of Rodinia, Pannotia and Gondwana supercontinents the East Antarctic shield represents a significant volume of supercontinents’ crust. Petrogenetic models for East Antarctic orthopyroxene granitoids became fundamental for any paleogeodynamic reconstractions of supercontinents. We collected structural geology, tectonics, and geochronology data for orthopyroxene granitoid intrusions belonging to orogens of certain age and combined with the plotting of geochemical data in major and rare element tectonic discrimination diagrams, also analyzed the Sm‒Nd isotope system data. The East Antarctic orthopyroxene granitoids tightly related to orogens are characterized by magmatic sources determined as combination of mafic lower crust, upper crust and juvenile mantle materials mixing up in different proportions depending on type of orogeny. It was established that orthopyroxene granitoids related to Mesoproterozoic and Late Mesoproterozoic‒Early Neoproterosoic orogens formed at different stages of long-lived collision transforming from ocean‒continent to continent‒continent types whereas Late Neoproterosoic‒Early Paleozoic orthopyroxene granitoids arose in post-collision tectonic setting.